The present invention relates to the field of elevator systems. In particular, the present invention relates to a power system for driving an elevator hoist motor from an irregular power source.
A regenerative drive for an elevator hoist motor typically includes a converter connected to an inverter via a DC bus. The inverter is connected to the hoist motor and the converter is connected to an AC power supply, such as from a power utility. When the elevator hoist motor is motoring, power from the AC power supply powers the converter, which converts the AC power to DC power for the DC bus. The inverter then converts the DC power on the DC bus to AC power for driving the hoist motor. In regenerative mode, the load in the elevator drives the motor so it generates AC power as a generator. The inverter converts the AC power from the hoist motor to DC power on the DC bus, which the converter then converts back to AC power for delivery to the AC power supply.
The drive is typically designed to operate over a specific input voltage range from the AC power supply. This range is commonly specified as a nominal operating voltage with a tolerance band (e.g., 480 VAC±10%). Thus, the components of the drive have voltage and current ratings that allow the drive to continuously operate while the AC power supply remains within the designed input voltage range. However, in certain markets the utility network is less reliable, where persistent utility voltage sags or brownout conditions (i.e., voltage conditions below the tolerance band of the drive) are prevalent. When utility voltage sags occur, the drive draws more current from the AC power supply to maintain uniform power to the hoist motor. In conventional systems, when excess current is being drawn from the AC power supply, the drive will shut down to avoid damaging the components of the drive. As a result, elevator service is unavailable until the AC power supply returns to the nominal operating voltage range.